Inorganic acicular bodies and method for producing the same

ABSTRACT

Acicular bodies of a metal compound are produced by slowly precipitating an organic salt of the metal from a solution of an ester of a dicarboxylic acid having 1-5 carbon atoms and firing the precipitate in an oxidizing atmosphere. These acicular bodies have a cross-sectional dimension less than about 20 μm and are useful for providing reinforcement of a larger ceramic body. Acicular bodies of rare-earth metal oxides also are useful in reinforcing x-ray scintillator bodies without diminishing their luminescent capacity.

BACKGROUND OF THE INVENTION

[0001] This invention relates to acicular bodies of inorganic materialsand a method for producing the same. More particularly, this inventionrelates to micron-sized acicular bodies of metal oxides that are usefulas reinforcements for ceramic composites and a method for producing thesame. This invention also relates to composite materials having acicularbodies of metal oxides as reinforcements.

[0002] Shaped bodies of ceramics have found uses in many applicationssuch as aeronautics, medical diagnostic systems, energy conversion,automotive components, and lighting. In some of these applications, theceramic bodies are typically sintered shaped bodies that are resistantto deterioration in a harsh environment, such as high-temperature orcorrosive environment. In other applications, such as in medicaldiagnostic systems, sintered bodies of certain ceramics, such as x-rayscintillators, are used for the unique properties of the chosen ceramicto luminesce upon being excited by a stimulating radiation. However,sintered ceramic bodies typically have smaller tensile stress thancompression stress and are prone to cracking. To impart increasedtoughness and fracture resistance, these bodies are often reinforcedwith other inorganic fibers, such as metal carbide, boride, nitride, oroxide.

[0003] One common method for producing inorganic fibers is disclosed inU.S. Pat. No. 5,686,368. In this process a fibrous metal oxide productis made by providing an fiber template made of an organic material suchas polyester, rayon, cellulose, etc.; soaking the fiber template toimpregnate it with a rare-earth nitrate; drying the impregnated fiber;and heating the impregnated fiber to burn out the organic template andto convert the rare-earth metal nitrate to rare-earth metal oxide. Theresulting product is a rare-earth metal oxide fiber having substantiallythe same shape and dimension as the organic fiber template.

[0004] U.S. Pat. No. 5,865,922 discloses a similar process for makingceramic fibers. In this process, fibers of a polymeric material arecoated with silicon carbide or nitride vapor at a very high temperatureto yield a partially rigid fibrous body. The coated fibers areinfiltrated with an organic resin material, then pyrolyzed to producefibers comprising porous carbon and silicon carbide or nitride. Theporous fibers are then infiltrated with liquid silicon to react with theporous carbon to yield fiber predominantly composed of silicon carbide.

[0005] In another process disclosed in U.S. Pat. No. 6,120,840; afibrous material is infiltrated with a polymer precursor of an organictransition metal complex. The polymer precursor is then cured anddecomposed to convert the organic transition metal complex to atransition metal boride or carbide.

[0006] Although good inorganic fibers could be obtained from theseprocesses, the manufacturing cost is undoubtedly high due to the use ofpossibly costly organic fiber raw materials and the complex multi-stepnature of the processes.

[0007] Therefore, there is a need for a simple process for makinginorganic fibers. It is also very desirable to produce inexpensiveinorganic fibers without resorting to using organic fiber templates.Furthermore, in many instances, it is very desirable directly to produceinorganic fibers that have the same composition as the ceramic matrix inwhich they will reside.

SUMMARY OF THE INVENTION

[0008] The present invention provides inorganic acicular bodiescomprising at least one inorganic compound of a metal selected from thegroup consisting of Groups IB, IIA, IIB, IIIA, IIIB, IVA, IVB, VA, VB,VIA, VIB, VIIA, VIIB, VIIIA, rare-earth metals of the Periodic Table,and mixtures thereof. These acicular bodies may be used in reinforcementof composite ceramic bodies. The inorganic acicular bodies may beproduced directly to have the same composition as the ceramic bodies.More particular, the inorganic acicular bodies comprise metal oxides andhave a cross-sectional dimension of less than about 20 μm. The inorganicacicular bodies are characterized in that their cross section isgenerally polygonal and their lengths are much longer than theircross-sectional dimension.

[0009] According to one aspect of the present invention, the inorganicacicular bodies are produced by a method comprising the steps of:preparing a solution of a precursor of the inorganic material; addingthe solution of the inorganic material into a solution of an ester of adicarboxylic acid; precipitating an organic salt of the ester of thedicarboxylic acid comprising the inorganic material (hereinafter calledthe “organic salt”) in acicular shape; separating the acicular-shapedbodies of the organic salt; drying the acicular-shaped bodies of theorganic salt; and firing the acicular-shaped organic salt in anoxidizing atmosphere to produce inorganic acicular bodies.

[0010] Other aspects, advantages, and salient features of the presentinvention will become apparent from a perusal of the following detaileddescription, which, when taken in conjunction with the accompanyingfigures, discloses embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a scanning electron microscopy (“SEM”) photograph ofacicular bodies of precipitate of yittrium gadolinium oxalate fromdimethyl oxalate solution.

[0012]FIG. 2 is a SEM photograph of acicular bodies of yttriumgadolinium oxide after firing.

[0013]FIG. 3 is SEM photograph of acicular bodies of precipitate ofyittrium gadolinium oxalate from diethyl oxalate solution.

[0014]FIG. 4 is an x-ray diffraction pattern of the yittrium gadoliniumoxide powder from fired acicular bodies of the present inventionindicating the Gd_(0.745)Y_(1.255)O₃ composition.

DETAILED DESCRIPTION OF THE INVENTION

[0015] The present invention provides a simple method for producinginorganic acicular bodies or fibers that may be used as reinforcementsin larger composite ceramic bodies comprising a ceramic matrix havingthese acicular bodies embedded therein. The acicular bodies of thepresent invention comprise crystals of a compound of a metal present inthe inorganic precursor material. The method of the present invention issuitable for producing acicular bodies comprising compounds of one ormore metals selected from Groups IB, IIA, IIB, IIIA, IIIB, IVA, IVB, VA,VB, VIA, VIB, VIIA, VIIB, VIIIA, and the rare-earth metals of thePeriodic Table. The method of the present invention is particularlysuitable for producing acicular bodies of oxides of metals of GroupsIIA, IIIA, IIB of the Periodic table, and oxides of rare earth metals.The method of the present invention is more particularly suitable forproducing acicular bodies of oxides of scandium, yttrium, lanthanum,aluminum, gallium, indium, thallium, cesium, praseodymium, neodymium,europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium,ytterbium, lutetium, and mixtures thereof. For example, acicular bodiesof yittrium gadolinium oxide of the present invention may be producedand incorporated in a europium- or praseodymium-doped yittriumgadolinium oxide ((Y,Gd)₂O₃) x-ray scintillator to provide reinforcingstrength to shaped bodies of the scintillator. Since the acicular bodiesfibers of the present invention may be made to have the same compositionas that of the scintillator bodies, the luminescence capacity of thesescintillators is not diminished when these fibers are incorporated intothe scintillator bodies, as would be with other types of fibers. Whenthe composition of the acicular bodies is the same as that of theceramic bodies in which they reside, another benefit may be realized inthat a stronger bond may develop between the two phases because of theircompatibility.

[0016] The inorganic acicular bodies of the present invention areproduced by a method comprising the steps of: (1) preparing a solutionof a precursor of the inorganic material; (2) adding the solution of theinorganic material into a solution of an ester of a dicarboxylic acid;(3) precipitating an organic salt of the ester of the dicarboxylic acid(the “organic salt”) in acicular-shaped bodies; (3) separating theacicular-shaped bodies of the organic salt; (4) drying theacicular-shaped bodies of the organic salt; and (5) firing theacicular-shape bodies of the organic salt in an oxidizing atmosphere ata temperature and for a time sufficient to convert the organic salt toinorganic acicular bodies. The production of inorganic acicular bodiesfrom this process was an unexpected result of the reaction of an esterof a dicarboxylic acid and an inorganic metal salt.

[0017] The solution of the precursor inorganic material may be anaqueous solution of a water-soluble inorganic precursor material or anacidic solution of the same when the solubilization of such precursor inthe solution need be enhanced. In some instances, an amount of a polarsolvent, such as a low-molecular weight alcohol having 1-3 carbon atoms,may be added into the solution to enhance the solubilization, as wouldbe recognized by a person skilled in the art. Any acid that is capableof dissolving the chosen inorganic precursor may be used to produce theacid solution. For example, hydrochloric acid, nitric acid, sulfuricacid, citric acid, or acetic acid may be used. The choice of acid isdetermined by the solubility limit of the inorganic precursor in theacid. Typically, the concentration of the inorganic precursor materialin the final acid solution is far from its solubility limit.

[0018] Esters of dicarboxylic acids that may be used in the presentinvention are methyl, ethyl, propyl, dimethyl, diethyl, and dipropylesters. Examples of dicarboxylic acids that may be used to form an esterfor the process of the present invention are oxalic acid, malonic acid,succinic acid, and glutaric acid. The preferred esters are dimethyl anddiethyl oxalate. The aqueous or acidic solution of inorganic precursoris slowly added into the dicarboxylic acid ester solution whilestirring. The size of the acicular bodies may be influenced by the rateof addition of the solution. For example, it has been known that thegrowth of large crystals is promoted by slow nucleation of the crystals.Therefore, a slower addition of the acid solution into the dicarboxylicacid ester solution may promote the formation of larger acicular bodies.

[0019] The precipitated, separated, and dried acicular bodies of theorganic salt are fired in an oxidizing atmosphere, such as air, oxygen,carbon dioxide, mixtures of oxygen and one or more inert gases selectedfrom the group consisting of helium, neon, argon, krypton, and xenon, ormixture thereof, at a temperature and for a time sufficient to decomposethe dicarboxylic acid ester. A temperature from about 400 to about 1400°C. is sufficient for this purpose. Preferably, the precipitate is firedat a temperature from about 700° C. to about 1000° C. The firing may beconveniently done in a batch-wise or continuous process. The firing timeis determined by the quantity of precipitate to be fired, the amount ofoxidizing gas conducted through the firing equipment, and the gas-solidcontact in the firing equipment. A firing time of about 2 to about 6hours is adequate to convert all the oxalate to oxide.

EXAMPLE 1

[0020] An acidic solution of Y₂O₃ and Gd₂O₃ in nitric acid was prepared.The solution contained 5 g Y₂O₃, 4.0 g Gd₂O₃, 19.7 g concentrated HNO₃solution (about 69.5 weight percent acid), and 37.1 g deionized water.The solution was stirred until all the oxides were visibly dissolved.

[0021] A dimethyl oxalate solution was prepared by adding and dissolving22.3 g dimethyl oxalate in 345 g of deionized water. The yittriumgadolinium oxide acidic solution was added drop-wise into the oxalatesolution while stirring with a magnetic stirring bar. An amount of 600 gof deionized water was added to the oxalate solution immediately afterall the acidic solution was added. Visible precipitate appeared in about2-4 hours. The solution was decanted and the precipitate was washed witheight 1000-ml portions of deionized water. After the eighth wash, the pHof the wash water was about 6.

[0022] The precipitate was filtered, dried overnight in an oven at 105°C., and then fired in air in a furnace at 750° C. for 4 hours.Alternatively, the drying may be carried out at any temperature abovethe boiling point of the solvent of the solution. The fired precipitatewas analyzed using SEM and x-ray diffraction, showing that theprecipitate were of acicular shape having generally a polygonal crosssection, a cross-sectional dimension of less than about 2 μm, and acomposition of Gd_(0.745)Y_(1.255)O₃.

EXAMPLE 2

[0023] The same procedure was repeated except 27.62 g of diethyl oxalatewas used in place of dimethyl oxalate. Visible precipitate appearedafter about 8 hours. The fired precipitate showed a mixture of acicularand plate-like bodies.

EXAMPLE 3

[0024] The same procedure as in Example 1 was repeated except 38.23 g ofdibutyl oxalate was used in place of dimethyl oxalate. No powderprecipitate was obtained in 5 days. Instead, the precipitate consistedof large clumps.

[0025] In another embodiment of the present invention, acicular bodiesof mixed rare-earth and Group III metal oxides may be produced by themethod of the present invention to be used as reinforcements forsintered shaped bodies of x-ray scintillators used in, for example,computed tomography scanning systems. Examples of such oxides, besideyittrium gadolinium oxide, are gadolinium gallium oxide, gadoliniumscandium gallium oxide, lutetium aluminum oxide, yittrium gallium oxide.These oxides may be doped with one or more rare earth metals of thelanthanum series, such as praseodymium, europium, ytterbium, samarium,or neodymium, as needed by providing a compound of the chosen metal inthe mixture of the inorganic precursor material so that the compositionof the final acicular bodies is the same as that of the scintillator.

[0026] In still another embodiment of the present invention, theacicular bodies have the alumina or alumina silicate composition and areused to reinforce sintered polycrystalline alumina or silicon carbidebodies that find wide applications in lighting systems or high-speedrotating machinery.

[0027] In one preferred embodiment, a low-melting inorganic matrixcontaining acicular bodies is extruded to produce a composite materialcomprising fiber bundles having a preferred alignment. Such a compositematerial may be useful in optical applications.

[0028] While various embodiments are described herein, it will beappreciated from the specification that various combinations ofelements, variations, equivalents, or improvements therein may be madeby those skilled in the art, and are still within the scope of theinvention as defined in the appended claims.

1-6. (canceled)
 7. A method for producing acicular bodies of at leastone inorganic compound of a metal comprising the steps of: preparing asolution of a precursor of said inorganic compound of said metal toobtain a first solution; preparing a solution of an ester of adicarboxylic acid to obtain a second solution; adding said firstsolution in increments into said second solution to form a mixedsolution and to obtain an acicular-shaped precipitate of an organic saltof said metal from said mixed solution; separating said precipitate fromsaid mixed solution; drying said separated precipitate; and firing saiddried precipitate in an oxidizing atmosphere at a temperature for a timesufficient to convert said organic salt of said metal to acicular bodiesof said inorganic compound of said metal; wherein said metal is selectedfrom the group consisting of Groups IB, IIA, IIB, IIIA, IIIB, IVA, IVB,VA, VB, VIA, VIB, VIA, VIIB, VIIIA of the Periodic Table, rare earthmetals, and mixtures thereof.
 8. The method according to claim 7,wherein said at least one compound of said metal is an oxide of saidmetal.
 9. The method according to claim 7, wherein said precursor ofsaid inorganic compound of said metal is soluble in water.
 10. Themethod according to claim 7, wherein said first solution is an acidicsolution comprising an acid selected from the group consisting ofhydrochloric acid, nitric acid, sulfuric acid, citric acid, acetic acid,and mixtures thereof.
 11. The method according to claim 7, wherein saiddicarboxylic acid is selected from the group consisting of oxalic acid,malonic acid, succinic acid, glutaric acid, and mixtures thereof. 12.The method according to claim 11, wherein said ester is selected fromthe group consisting of methyl, ethyl, propyl, dimethyl, diethyl,dipropyl esters, and mixtures thereof.
 13. The method according to claim7, wherein said ester is selected from the group consisting of dimethyloxalate, diethyl oxalate, and mixtures thereof.
 14. The method accordingto claim 7, wherein said inorganic compound of said metal is an oxide ofsaid metal.
 15. The method according to claim 7, wherein said drying iscarried out above a boiling point of a liquid of said mixed solution fora time sufficient substantially to remove a liquid from saidprecipitate.
 16. The method according to claim 7, wherein said firing iscarried out at a temperature from about 400° C. to about 1400° C. 17.The method according to claim 7, wherein said firing is carried out forabout 4 hours.
 18. The method according to claim 7, wherein saidoxidizing atmosphere is selected from the group consisting of air,oxygen, carbon dioxide, mixtures of oxygen and at least one inert gas,and mixtures thereof.
 19. A method for producing acicular bodies of atleast one inorganic compound of a metal comprising the steps of:preparing a solution of a precursor of said inorganic compound of saidmetal to obtain a first solution; preparing a solution of an ester of adicarboxylic acid to obtain a second solution; adding said firstsolution in increments into said second solution to form a mixedsolution and to obtain an acicular-shaped precipitate of an organic saltof said metal from said mixed solution; separating said precipitate fromsaid mixed solution; drying said separated precipitate; and firing saiddried precipitate in an oxidizing atmosphere at a temperature for a timesufficient to convert said organic salt of said metal to acicular bodiesof said inorganic compound of said metal; wherein said metal is selectedfrom the group consisting of Groups IIA, IIIA, IIIB of the PeriodicTable, rare earth metals, and mixtures thereof.
 20. A method forproducing acicular bodies of at least one inorganic compound of a metalcomprising the steps of: preparing a solution of a precursor of saidinorganic compound of said metal to obtain a first solution; preparing asolution of an ester of a dicarboxylic acid to obtain a second solution;adding said first solution in increments into said second solution toform a mixed solution and to obtain an acicular-shaped precipitate of anorganic salt of said metal from said mixed solution; separating saidprecipitate from said mixed solution; drying said separated precipitate;and firing said dried precipitate in an oxidizing atmosphere at atemperature for a time sufficient to convert said organic salt of saidmetal to acicular bodies of said inorganic compound of said metal;wherein said metal is selected from the group consisting of scandium,yttrium, lanthanum, aluminum, gallium, indium, thallium, cesium,praseodymium, neodymium, europium, gadolinium, terbium, dysprosium,holmium, erbium, thulium, ytterbium, lutetium, and mixtures thereof. 21.A composite ceramic body comprising a ceramic matrix and acicular bodiesof at least one inorganic compound of a metal selected from the groupconsisting of Groups IB, IIA, IIB, IIIA, IIIB, IVA, IVB, VA, VB, VIA,VIB, VIA, VIIB, VIIIA of the Periodic Table, rare earth metals, andmixtures thereof; said acicular bodies being embedded in said ceramicmatrix.
 22. A composite ceramic body comprising a ceramic matrix andacicular bodies of at least one inorganic compound of a metal selectedfrom the group consisting of Groups IIA, IIIA, IIIB, IVA of the PeriodicTable, rare earth metals, and mixtures thereof; said acicular bodiesbeing embedded in said ceramic matrix.
 23. A composite ceramic bodycomprising a ceramic matrix and acicular bodies of at least oneinorganic compound of a metal selected from the group consisting ofscandium, yttrium, lanthanum, aluminum, gallium, indium, thallium,cesium, praseodymium, neodymium, europium, gadolinium, terbium,dysprosium, holmium, erbium, thulium, ytterbium, lutetium, and mixturesthereof; said acicular bodies being embedded in said ceramic matrix. 24.The composite ceramic body according to claim 23, wherein said ceramicmatrix and said acicular bodies have a same composition.
 25. Thecomposite ceramic body according to claim 24, wherein said compositeceramic body comprises a scintillator of a computed tomography x-raysystem.
 26. A composite ceramic body comprising a polycrystallinealumina matrix and acicular bodies of alumina embedded therein.
 27. Acomposite ceramic body comprising a polycrystalline silica matrix andacicular bodies of silica embedded therein.
 28. A light pipe fortransporting light, said light pipe comprising acicular bodies of atleast one scintillating material.
 29. A composite ceramic bodycomprising a ceramic matrix and acicular bodies of a least one inorganiccompound, wherein said acicular bodies substantially align in directionof their longer axes.